Methods and apparatuses for fill level measurement, which work according to the sounding principle, are sufficiently known. For example, described in the documents, German patents, DE 21 51 094, DE 24 01 486 B2, DE-patent 819 923, and DE 39 42 239 A1; U.S. Pat. No. 3,838,518, German patents, DE 195 43 352 A1, G 70 31 884.2, DE-patent 819 923, G 73 29 766.2, and DE 19730196 A1; as well as German patent, DE 28 53 360 A1 are fill level measuring systems for highly accurate fill level determination based on the sounding principle. In the case of these methods for fill level measurement according to the sounding principle, a plumb bob hanging on a measuring line descends toward the fill substance, respectively bulk good. Upon striking the fill substance, the length of the measuring cable wound off of the cable drum is ascertained and the fill level height, respectively the fill quantity, displayed on a display device. For different fill substances, differently suitable plumb bobs are applied.
The main field of application of electromechanical sounding is for fill level measurement in the case of very high containers, where solutions with other measuring principles are very costly or not possible because of physical reasons. With electromechanical sounding, fill levels in containers are measurable currently up to, for instance, 70 m in height with an accuracy of under a millimeter.
Other apparatuses for liquid level measurement and density determination, which work according to the displacement measuring principle, are known from German patents, DE 37 21164 A1, DE2853360A1, DE 2401486 B2 and DE 2659416 A1.
Known from DE 37 21164 A1 is a fill-level measuring device, which contains a float on a wire, which floats on the surface of a liquid (not shown). The wire is wound on a drum and can be wound onto or off of the drum by means of the rotational movement of a measuring shaft connected with the drum. Connected with the base of the drum is a measuring shaft. If the liquid level, on which the float floats, changes, then also the stress exerted by the wire on the drum changes therewith. This change of the stress exerted by the wire is converted via an outer magnetic ring acting as a coupling part into a torque of the measuring shaft. The cylindrical, outer magnetic ring is connected with the base in the interior of the drum. Magnetic poles, south- and north poles, are arranged alternately in the circumferential direction of the outer magnetic ring. Alternately embodied on the inner magnetic ring connected with the measuring shaft are magnetic north- and south poles in number equal to those of the outer magnetic ring. An electromagnetic transducer, e.g. a Hall element, is arranged on the outer periphery of the inner magnetic ring in the boundary region between different magnetic poles. If there is produced in the case of a change of the liquid level to be measured a force, which causes a relative movement between the outer and inner magnetic rings, then a change of the magnetic flux present between the outer and inner magnetic rings effects in the electromagnetic transducer an electrical signal, by which the measuring shaft is so rotated that the relative movement between the inner and outer magnetic rings is brought back to zero and, from this, a measured value of the achieved liquid level is won.
In order to calibrate the weight measuring system in the case of such electromechanical fill level measurement devices, in a calibration mode, a known, freely hanging weight, respectively a displacement element not located in the medium, with a defined weight is hung on the measurement wire and its weight ascertained. However, measurement errors can occur in the calibration mode due to tension forces from the attaching of the displacement element or the weight to the measurement wire or due to tribological properties of the measurement mechanism.